Dynamic test apparatus for piezoelectric crystals



D. W. COLE ET AL Filed Jan. 22, 1947 Osct'ZZaSc'qoe DYNAMIC TEST APPARATUS FOR PIEZOELECTRIC CRYSTALS Aug. 8, 1950 Patented Aug. 8, 1950 DYNAMIC TEST APPARATUS FOR PIEZELECTRIC CRYSTALS Dana W. Cole, Buialo, Roy A. Gull, Kenmore, `lohn .0. Bontius, Williamsville, and .ohn P.

Rug, Buffalo, N.y Y., `assignors Generation, Bualo, N. Y.

Application January 22, 1947, Serial No. 723,478

(Cl. 1l`5-l83) 7 Claims. 1k

This invention relates to test .apparatus .for piezoelectric crystals, vand more particularly to apparatus `for predetermining vthe .performance f -such crystals -in 5signal-seeking receivers.

Among the objects -of our invention may be mentioned the following:

To provide apparatus vby Nmeans -of-which the piezoelectric -crystal =voltalge rise time may be measured directly.

To-provide apparatus by Vmeans of which the crystal voutput voltage is readily determinable under dynamic rather-than static conditions.

To provide apparatus -by whichrthe measurev@ments -may be'made atthe resonant frequency of the crystal.

'To provide apparatus in which differenttypes and cuts of crystals may be quickly compared andfthe characteristics of the crystals determined, and to provide apparatus Ywhich may be utilized for incoming-inspection lof crystals Afor production.

lreceiver using a piezoelectric crystal,

Fig.` 2 `is a schematic diagram ofA one form of test apparatus in accordance with our'invention, and

y-Figs 3aandSb are curves explaining the operation ofour invention.

In order to facilitate an understanding of o ur invention, it isfirstnecessarytounderstand in a general way the operation of-a signal-seeking A`receiver.

Referring now more particularly to IFig. 1,1!!

ldesignates theantenna coupled through coupling coils IliaV and IIb toradio frequency amplifier- =converter and oscillator l2, the output'o'fywhich 'vis' supplied to intermediate frequency amplifier |13, the output of which in turn issupplied to detector and audio amplifier M, andthence to Vloud-*speaker I5. In theforrn herein shown the lreceiver will be'recognized as of thesuperhetero- 'dyne type particularly adaptedV for` broadcast reception. 1 In order to provide for operation as a signalvseekingV receiver, a portion of the youtput of 2 I. F. amplifier i3 is` supplied to limiter i6, which is .preferably set to limit at the maximum value of the weakest signal desired to be received. The circuits up to this point have a band Width suicient to accommodate the usual broadcast transmission; i.,e., at least 5000 cycles.

The output of limiter i6 is then sup-plied to crystal filter I1, and ,theband width of filter i7 is preferably only va small fraction of the band .width of the `preceding circuits; for instance, lter lil mayhave a band pass width of only 500 cycles. The rectified voltage output from the crystal lter is impressed between the control electrode and cathode oftube I8, which contains Yin its platecircuitrelay `I8 controlling the circuit of tuner motor 2l energized by suitable source of power 129, and indicated as driving the tuning instrumentality condenser 22: connected across coil 23 in the oscillator tuning circuit.

.Control tube Y[8 and relay I9 are so arranged ythat in the absence of a received signal no voltage appears on control tube I8, and the relay will be in position closing the motor circuit. Assuming ythe receiver` to be operating, tuner motor 2l will operate, driving tuning instrumentality 22 and scanning thereceiver spectrum.

When a Vsignalis tuned in, a voltage is developed across .crystal lter Il, which is applied to .controlitube .I8 to cause relay i9 to open the motorlcircuit, .whereupon the receiver will stop scanning and will remain tuned to the incoming signal as long as it is received, or until the operator starts the scanning process by a switch (not shown).

Since theconstruction and .operation of signal-seeking. receivers is per se no part of this invention,theconstruction and operation is described only tothe foregoing extent, and only becausesome understanding of the action of such ya receiver isnecessary to a` complete understandingof the invention herein claimed.v

In theoperation of various signallseeking receivers it .wasdiscovered that the characteristics of the 'crystal employed had a marked eiiect on the scanning speed.` which could be employed. It is .desirablethat this scanning speed be a maximminorder to avoid delay in changing tuning. However, it was found that the permis- -siblescanningfs eed varied considerably with difere'ntcrystals.

.For instance, with one crystal it might be found thata scanning time of 5 seconds for complete scanningironi one end of the spectrum to thefcther was lthe shortest'that ,couldbe employed. Attempts to operate the kmotor at a 3 faster rate would result in stations being missed. Then, substituting another crystal tuned to the same frequency, it might be found that the scanning speed could be increased, say, to 4 seconds, or it might equally be found that it might have to be reduced to 6 or 7 seconds.

By such a out and try method, it is possible to classify various types and cuts of crystals, but it is apparent that this method is entirely inadequate for production purposes and that some method of selecting crystals according to stopping time (the development of a resonant voltage sufficient to operate the control tube) was necessary in order that uniform scanning speed might be provided, it being obviously impractical to adjust the scanning speed in each receiver to the particular crystal employed there- Consideration of the operating requirements shows that, to be satisfactory, the crystal must meet two requirements: (1) It must possess adequate selectivity characteristics in order that the control voltage may be developed at the precise point of receiver resonance (front end tuning), and (2) the crystal must develop sufficient voltage to excite the relay control tube grid, and this voltage must develop very rapidly with the start of crystal excitation.

It will be understood that crystal selectivity may be measured under static conditions, but this will give no information on the rate of voltage rise and the maximum voltage developed, because these latter vary as a function of the scanning speed for any particular crystal.

By the use of the apparatus now about to be described, we have found that it is possible to measure each crystal with respect to. the Voltage developed and the time-required for the voltage to rise to any particular fraction of its final value. to inspect a considerable number of crystals in a relatively short time and to discard those Whose performance is not satisfactory.

As an example, suppose that all receivers are to be built for a scanning speed of seconds;

that is, an elapsed time of 5 seconds for the receiver to tune from one end of its spectrum to the other. Using our apparatus we are able to test the group of crystals and discard any whose rise time is slower than that required for 5 seconds scanning speed.

The apparatus in accordance with our invention consists of radio frequency signal generator 32 of any well known construction available on the market commercially delivering .oscillations of the crystal resonant frequency, in the present instance 455 kc. Signal generator 32 feeds pulse generator such as the Colonial Model '700 pulse generator advertised and described in the periodical Electrical Equipment for June 1944 or similar apparatus available on the market, shaper such as shown in U. Si. Patent 2,115,881 to Rossenstein, and radio frequency modulator 33 of any well known construction. Pulse generator, shaper, and radio frequency modulator 33 is preferably adjustedto give pulses of from about 1.0 to about 20.0 milliseconds duration.

Audio oscillator is provided, which may be variable and adjustable from approximately 100 to 5000 cycles per second, and this oscillator constitutes the master time base of the apparatus, all other necessary time functions. being synchronized either directly or indirectly with it, and it is adjusted to an appropriate frequency By the use of this apparatus we are able f.;-

for producing time markers or pips on the oscilloscope trace of, for example, 500 or 1000 cycles per second (2.0 or 1.0 milliseconds repetition rate). These pips will be observed on the oscilloscope trace, shown both in Figs. 2 and 3, each representing in this particular instance an elapsed time of 1.0 to 2.0 milliseconds.

Square wave generator 3l of any suitable type such as those available commercially on the market is preferably synchronized at a submultiple frequency of the oscillator in the range of 10 to 50 cycles per second. The pulse generator, shaper, and radio frequency modulator are preferably adjusted to modulate the output of the radio frequency signal generator which is tuned to the resonant frequency of the crystal under test, resulting in a series of pulses containing the resonant frequency of the crystal with a very steep wave front; i. e., of from 1 to 20 milliseconds duration, and repeated at the square wave generator frequency. 1

The pulse modulated radio frequency signal (square-Wave modulated radio frequency signal) is then applied to crystal network 36, containing the crystal under test, and the voltage developed by crystal rectifier 3l is applied to the vertical deflection input terminals of the oscilloscope, together with the time markers developed by the marker pulse generator 34 which may be another Colonial Model 700 pulse generatorv already mentioned, or othersuitable apparatus available commercially of 40-60 microsecondsduration, and repeated at 1.0 or 2.0 millisecond intervals. The horizontal sweep generator of the oscilloscope is adjusted to the square wave gen, erator frequency and synchronized directly with it If the crystal is short-circuited and no other reactive elements are included in the circuit, the pulse image on the oscilloscope will have the same characteristics as the initiating pulse and will be as shown in Fig. 3a; that is, it will have a steep Wave front, flat top, and rapid decay.

If, now, the short-circuit is removed from the crystal, the leading edge of the pulse will now change in slope with an increased rise time, and similarly the trailing edge of the ,pulse will show an increased decay time. The change is exemplified in Fig. 3b. `If, the pips shown on both Figs. 3a and 3b and connected by dotted lines are 2.0 milliseconds apart, the crystal whose curve is shown in Fig. 3b is shown to have a rise time of resonant voltage of 10 milliseconds to amplitude and anvequal decay time to zero amplitude. l

Also, it will be noted that this particular crystal rises to 50% Voltage in` 2.0 milliseconds and decays to 50% voltage in 2.0 milliseconds, and rises to 75% in 4.0 milliseconds and decays to 75% in 4.0 milliseconds. lSome crystals will rise faster than this and may rise to 100% amplitude in a very few milliseconds and decay to 100% in the same time, while other crystals may be so slow that they do notproduce 100% output during the :pulse duration. This would be indicated bya failure of the curve in Fig. 3b to achieve zero slope before decay starts, giving the curve a more sawtooth or peaked effect. y

, Thus it will be seen that by adjusting the crystal (signal) pulse duration to a known value long enough for the crystalto reach maximum output, and by reading the number of time markers whose repetition rate is known, the time required for any crystalto reach any percentage of maximum output may be readily andquickly Thev frequencyof audio A Ioscillator" 3D' 'fcycles' pery second Square cycles per second (1,/ of the Amaster vtime base frequency) sigaai'puise curar-roher pll's' "generator '33 Frequency of radio frequency generator 32 12 milliseconds' 455 kc. (for crystals supposed to be resonant at 455 kc.)

Marker pulse generator 34 60 microseconds pulse duration, repetition rate 500 cycles per second, one time marker every 2.0 milliseconds.

Oscilloscope sweep generator of oscilloscope 35 Synchronized at 20 cycles per second With the square wave generator With the crystal short-circuited, the square wave generator and the horizontal sweep oscillator are adjusted for exact synchronism with the audio oscillator; that is, until both the time marker` pulses are stopped on the oscilloscope screen. The trace will then have the appearance shown in Fig. 3a. When the crystal short-circuit is removed, the trace may then take the form shown on the screen of oscilloscope 35, as shown in Fig. 2, and as shown to a larger scale in Fig. 3b.

We have here referred to rectifying the output of the crystal before applying it to the oscilloscope. This is not absolutely necessary, as the radio frequency output may be applied, but in general it is better to rectify, as a simpler oscilloscope suffices if the output is first rectified.

In the specication we have explained the principles of our invention and the best mode in which we have contemplated applying those principles, so as to distinguish our invention from other inventions; and we have particularly pointed out and distinctly claimed the part, improvement, or combination which we claim as our invention or discovery.

While we have shown and described certain preferred embodiments of our invention, it will be understood that modifications and changes may be made without departing from the spirit and scope thereof, as will be clear to those skilled in the art.

We claim:

1. The method of testing the time of response of a piezoelectric crystal, which comprises the step of repeatedly applying to the crystal a pulse modulated radio frequency signal of the resonant frequency of the crystal, having a steepv were front, simultaneously producing time marker pulses and producing a visible trace of .the voltage output of said crystal and of said time `marker pulses superimposed on each other.

2. The method of testing piezoelectric crystals, which. comprises generating a .radio frequency signal of the crystal resonant frequency. generating an audio frequency oscillation, producing from said oscillation square waves of a submultiple of the oscillation frequency, pulse modu-v lating said radio frequency signal with said submultiple ,square Waves, producing time marker pulses from said audio oscillation, impressing said modulated radio frequency signal on the crystal to be tested, and producing a visible trace of the voltage output of said crystal and said time marker pulses superimposed on each other.

3. The method of testing piezoelectric crystals, which comprises generating a radio frequency signal of the crystal resonant frequency, generating an audio frequency oscillation, producing from said oscillation square waves of a submultiple of the oscillation frequency. pulse modulating said radio frequency signal with said submultiple square waves, producing time marker pulses from said audio oscillation, impressing said modulated radio frequency signal on the crystal to be tested, rectifying the output of said crystal and producing a visible trace of the rectied output of the crystal and of said time marker pulses superimposed on each other.

4. Testing apparatus, comprising, in combination, a radio frequency signal generator` of the resonant frequency of the subject to be tested, an audio oscillator, a time marker pulse generator synchronized with said audio oscillator, a square wave generator synchronized with a submultiple frequency of said audio oscillator, a pulse generator synchronized with said square wave generator, and means for pulse modulating the output of said radio frequency signal generator by the output of said square wave generator means for applying the pulse modulated radio frequency signal to the subject to be tested and means for producing from the output of the subject to be tested a visible trace of the output voltage therefrom combined with the time marker pulses from said time marker pulse generator.

5. Piezoelectric crystal testing apparatus. comprising, in combination, a radio frequency signal generator of the resonant frequency of the crystal to be tested, an audio oscillator, a time marker pulse generator synchronized with said audio oscillator, a square wave generator synchronized with a submultiple frequency of said audio oscillator, a pulse generator synchronized with said square Wave generator, means for pulse modulating the output of said radio frequency signal generator by the output of said square wave generator, and means for impressing the pulse modulated radio frequency signal upon the crystal to be tested.

6. Piezoelectric crystal testing apparatus, comprising, in combination, a radio frequency signal generator of the resonant frequency of the crystal to be tested, an audio oscillator, a time marker pulse generator synchronized with said audio oscillator, a square wave generator synchronized with a submultiple frequency of said audio oscillator, a pulse lgenerator synchronized with said square wave generator, means for pulse modulating the output of said radio frequency signal generator by the output of said square wave generator, means for impressing the pulse audio "oscillator, a pulse .generator synchronized l With'lfsaid square `Wave` generator, means for `in odulating the output of vsaid signal generator by the ,outputof said square Wave generator to produce radio frequency pulses Vhaving a steep Wave front, and means for applying said vpulses tothe crystal to be-tested. i ,Y

DANA W. COLE. Bora-GUM.

l JOHN C KPONTIUS.

JOHN P. RUG.

REFERENCES l CITED The following referencesare of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,400,895 Wachtman May 28, 1946 2,408,858 Keizer Oct. 8, 1946 2,453,532 Norton Nov. 9, 1948 2,477,615 Isbisterfv .Q Aug. 2, 1949 

